Have you done anything with the coolant lately? replaced it? topped it up? found a nice little puddle under the car?

Could be 2 things -
* Low on coolant from a leak or not filling up properly, creates an air pocket in the system which contributes to overheating. Doesn't happen straight away so might have been a gradual thing. Once the car is completely cooled (!!!) take off the cap & see where the coolant level is. If it's low you'll need to fill up & burp the system.
* Thermostat has been bumped/damaged somehow when doing the gearbox & giving dodgey readings.